Radiative transfer of emission lines with non-Maxwellian velocity distribution function: Application to Mercury D2 sodium lines

We describe the theory and the numerical model used to simulate Doppler-broadened resonant emission lines for any type of velocity function distribution. The field of application of this theoretical model of radiative transfer is particularly well suited for the study of weakly dense atmospheres which are far from local thermodynamic equilibrium (as is the case for most planetary upper atmospheres/exospheres). This model is applied to study the potential effects of radiative transfer and non-Maxwellian distributions on the spectral shape of the D2 sodium emission line in Mercury’s exosphere. The small (but not negligible) optical thickness of the D2 sodium emission of an exosphere like Mercury’s (with a peak optical thickness of ∼2) can result in an increase of the observed spectral width by up to a few tens of percent. Combined with the non-Maxwellian nature of the exospheric velocity distribution, it may lead to an increase in the spectral width by a factor of up to 2 with respect to the width of an optically thin emission and a Maxwellian distribution. This model has been used to analyze new THEMIS observations of Mercury’s exosphere obtained at very high spectral resolution in a companion paper (Leblanc, F., Chaufray, J.-Y., Doressoundiram, A., Berthelier, J.-J., Mangano, V., Lopez-Ariste, A., Borin, P. [2013]).